The present invention relates to a sensor for reading original documents and used in facsimile devices, word processors, etc.
As disclosed in Japanese Patent Publication JP-A 60-244062, a conventional image sensor includes a first photoconductive cell (element) formed at a position where light for reading is received, and a second photoconductive cell (element) formed at a position where no light is received, both elements being connected in series, across which a voltage is applied in common and the potential at the junction point of the elements being extracted to the outside.
Another conventional image sensor, as described in Nikkei Electronics, 434 (1987), Vol. 207, pp. 207-221, includes a photoconductive element, a transfer switch and a reset switch each including a thin film transistor made of hydrogenated amorphous silicon (a-Si: H) or amorphous silicon (hereinafter referred to as a-Si), signal matrix leads and gate leads. So-called contact type reading is performed by placing an original document in contact with a transparent substrate over which photoconductive elements are formed, illumining the document through the transparent substrate with a light source disposed on the opposite side of the substrate from its surface over which the photoconductive elements are formed, and directing the reflected light to the photoconductive elements for photoelectric conversion.
The photoconductive elements receive only the light reflected by the original document and the produced photocurrent is stored in the storage capacitor for a predetermined time. A transfer signal is delivered to the gate lead to transfer signal electric charges produced by 32 photoconductive elements per block to the capacitor for 32 signal matrix leads using the switching operations of the a-Si thin-film transistors and the external circuit reads the signal voltages sequentially. Since transfer and resetting of signal electric charges per block are performed by supplying a signal externally to the gate leads, the gate leads used are the same in number as the blocks. The elements in a block are the same in number as the signal matrix leads.
The above two conventional techniques allow for variations in the illuminance of the light source illumining the document, which variations directly bring about corresponding undesirable variations in the output signal. Changes in the sensitivity due to temperature and aging and/or variations in the pattern accuracy lead directly to variations in the signal.
The latter conventional technique allows for the fact that because a photocurrent flowing through the photoelectric conversion element is all stored in a storage capacitance connected with a thin film transistor having a small conductance, it takes long time for transfer of the electric charges and hence high speed reading is difficult.
In the latter conventional technique, matrix driving for reading is all performed by an external circuit which drives a gate line and a matrix line, increasing the number of leads and disadvantageous for miniaturization and the external circuit is undesirably large-scaled.
Also, in the latter conventional technique, the elements of one block are the same in number as the signal matrix leads, so that a data transmission time and a reset time are required in block switching until the next block is read. Thus, the conventional technique does not allow for intervention of a rest time between signals for the adjacent blocks and signal reading is complicated.